Particle and device for image display

ABSTRACT

In particles for image displaying used in an image display device, in which one or more kind of particles are sealed between opposed substrates, at least one substrate being transparent, and, in which the particles are made to fly and move so as to display an image, each particle has a micro-irregularity at its surface. Moreover, in an image display device, in which one or more kind of particles are sealed between opposed substrates, at least one substrate being transparent, and, in which the particles, to which an electrostatic field produced by two kinds of electrodes having different potentials is applied, are made to fly and move so as to display an image, use is made of each particle having a micro-irregularity at its surface.

TECHNICAL FIELD

The present invention relates to an image display device, whichcomprises an image display panel enables to repeatedly display or deleteimages accompanied by flight and movement of particles utilizingCoulomb's force and so on.

BACKGROUND ART

As an image display device substitutable for liquid crystal display(LCD), image display devices with the use of technology such as anelectrophoresis method, an electro-chromic method, a thermal method,dichroic-particles-rotary method are proposed.

As for these image display device, it is conceivable as inexpensivevisual display device of the next generation from a merit having widefield of vision close to normal printed matter, having smallerconsumption with LCD, spreading out to a display for portable device,and an electronic paper is expected.

Recently, electrophoresis method is proposed that microencapsulatedispersion liquid made up with dispersion particles and colorationsolution and dispose the liquid between faced substrates. However, inthe electrophoresis method, there is a problem that a response speed isslow by the reason of viscosity resistance because the particles migrateamong the electrophoresis solution. Further, there is a problem oflacking imaging repetition stability, because particles with highspecific gravity of titanium oxide is scattered within solution of lowspecific gravity, it is easy to subside, difficult to maintain astability of dispersion state. Even in the case of microencapsulating,cell size is diminished to a microcapsule level in order to make it hardto appear, however, an essential problem was not overcome at all.

Besides the electrophoresis method using behavior in the solution,recently, a device without using the solution is proposed, in which twoor more groups of particles having different colors and different chargecharacteristics are sealed between two substrates and the particles, towhich an electrostatic field is applied, are made to fly and move so asto display an image. [The Imaging Society of Japan “Japan Hardcopy '99”(Jul. 21-23, 1999) Transaction Pages 249-252] Since this device is a drytype as opposed to the electrophoresis method, there is a merit suchthat a moving resistance of the particles is small and thus a responsespeed is fast.

The dry-type display device mentioned above has an operation mechanismsuch that a mixture of two kinds of the particles having differentcolors and different charge characteristics is sandwiched by electrodeplates and an electric field is generated between the electrode platesby applying a voltage to the electrode plates, thereby flying thecharged particles having different charge characteristics in a differentdirection to obtain a display element.

As to forces applied to the particles, there are an attraction forcebetween the particles due to Coulomb' force, an imaging force withrespect to the electrode panel, an intermolecular force, a liquidbridging force and a gravity.

When the force applying to the particles due to the electric field islarger than total forces mentioned above, the particle fly occurs.

In the case that an actual drive circuit in the image display devicetakes into consideration, it is preferred that the drive voltage islowered. As a large factor for defining the drive voltage, there aremainly an intermolecular force and a liquid bridging force. If thesefactors are made to be lowered, a decrease of the drive voltage isinstantly achieved. Therefore, it is very important to improve thesecharacteristics.

Moreover, in order to obtain god operation properties, it is preferredthat an electrostatic property is made to be higher to some extent.Therefore, it is another task to obtain the particles having anexcellent electrostatic property. The electrostatic property of theparticle itself for the image display is the most important factor whena force generated by applied electric field and an adhesion forcebetween the particles or to the substrate are controlled. However, theelectrostatic property of the particle is normally under control of amaterial of the particle itself, and thus it is difficult to control itaccurately by the particle itself.

Further, in the case that fine particles are used in a display element,it is necessary to use white color particles and black color particlesso as to make a contrast on color tone clear. Contrary to this,polymerized fine particles obtained from a general-purpose resin show anachromatic color, but, since they are fine particles, they look like awhite color by an irregular reflection of light. However, in the casethat they are used with the black particles in the image display device,a contrast ratio lacks and thus whiteness is decreased.

In order to improve the whiteness, there is a method such that whitefine powders such as titanium oxide powders or zinc oxide powders areincluded in the particle. However, in the case that the white fineparticles are to be included during a particle polymerization by meansof a polymerized method, it is necessary to control a surface affinityby performing a coupling agent treatment with respect to the white finepowders, so that the white fine powders are included in the particleeffectively. Therefore, the producing method becomes very complicated.

Further, in the case that the method mentioned above is utilized, it isnormally difficult to control a particle size. For example, a method,such that the particles are produced by kneading, grinding andclassifying a mixture of a main resin and white fine powdersconstituting the particle, is proposed. However, in this case, it is notpossible to obtain the particles having a narrow particle sizedistribution if the classifying operation is not performed. Moreover,since a mechanical grinding is performed so as to obtain the fineparticles, it is difficult to obtain the particles having a particlesize of 8 μm or less even if the fine particles having a particle sizeunder the above level are to be obtained.

DISCLOSURE OF INVENTION

The present invention is achieved by investigating the above problemsand has for its object to provide a particle and device for an imagedisplay, which can achieve a low electrostatic property, sufficientlyapply an electrostatic property to the particle and stably obtain anexcellent image having a sufficient contrast, in a dry-type imagedisplay device wherein the particle is flown and moved.

As a result of the inventor's dedicated investigation, it is found thatit is possible to obtain the particle having an excellent electrostaticproperty and achieving a decrease of the drive voltage by arranging amicro-irregularity on a surface of the particle, and, that it ispossible to obtain stably the excellent image having a sufficientcontrast since an incident light looks like a white color when viewingdue to its irregular reflection, and the present invention is realized.

That is, the present invention provides the following particles anddevice for the image display.

-   1. Particles for image displaying used in an image display device,    in which one or more kind of particles is sealed between opposed    substrates, at least one substrate being transparent, and, in which    the particles are made to fly and move so as to display an image,    characterized in that each particle has a micro-irregularity at its    surface.-   2. The particles for image displaying according to the above 1,    wherein a product of a specific surface area S (m²/g) and an average    particle diameter d(0.5) (μm) of the particles: (S×d(0.5)) is 10 or    more.-   3. The particles for image display according to the above 1 or 2,    wherein the average particle diameter d(0.5) is in the range of    0.1-50 μm.-   4. The particles for image displaying according to one of the above    1-3, wherein the particles is produced by including a volatile    component in an inner portion of the each particle at polymerization    and removing the volatile component by heating after polymerization.-   5. The particles for image displaying according to one of the above    1-3, wherein the particles is produced by crashing or sliding a    substance having a high stiffness with respect to the surface of    polymerized particles so as to form the micro-irregularity on the    surface of the each particle.-   6. The particles for image displaying according to one of the above    1-3, wherein the particles are produced by arranging a polymerized    portion having an indefinite shape on the surface of the polymerized    particles obtained by a suspension polymerization, by means of a    graft polymerization method.-   7. The particles for image displaying according to one of the above    1-3, wherein the particle is produced by adhering small child    particles on the surface of large mother particles.-   8. An image display device, in which one or more kind of particles    are sealed between opposed substrates, at least one substrate being    transparent, and, in which the particles, to which an electrostatic    field produced by two kinds of electrodes having different    potentials is applied, are made to fly and move so as to display an    image, characterized in that use is made of particles having a    micro-irregularity at their surfaces.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic view explaining one embodiment of a display methodin an image display device according to the invention.

FIG. 2 is a schematic view explaining another embodiment of a displaymethod in an image display device according to the invention.

FIG. 3 is a schematic view explaining a structure of an image displaydevice according to the invention.

BEST MODE FOR CARRYING OUT THE INVENTION

In an image display device according to the invention, one or more kindof particles are sealed between opposed substrates, at least onesubstrate being transparent, and the particles are made to fly and moveby means of Coulomb's force and so on so as to display an image.

The dry-type image display device mentioned above can be applied to adisplay method wherein the different particles having two or more colorsare moved in a vertical direction with respect to the substrate as shownin FIG. 1 and a display method wherein the powders having one color aremoved in a horizontal direction with respect to the substrate as shownin FIG. 2. For the sake of safety, the former display method ispreferred.

FIG. 3 is a schematic view explaining a structure of the image displaydevice. In the embodiment shown in FIG. 3, the image display devicecomprises opposed substrate 1 and substrate 2 and particles 3, andpartition walls 4 are arranged according to need.

As to the substrate, at least one of the substrate 1 and 2 is atransparent substrate through which a color of the particles can beobserved from outside of the device, and it is preferred to use amaterial having the high transmission factor of visible light and anexcellent heat resistance.

As the image display device, whether flexibility is necessary or not issuitably selected in accordance with its use. For example, it ispreferred to use a material having flexibility for the use ofelectronic-paper and so on, and it is preferred to use a material havingno flexibility for the use of a display of portable device such asmobile phone, PDA, laptop computer and so on.

Examples of the substrate material include polymer sheets such aspolyethylene terephthalate, polyether sulfone, polyethylene, orpolycarbonate, and inorganic sheets such as glass, quartz or so.

The thickness of the substrate is preferably 2 to 5000 μm, morepreferably 5 to 1000 μm. When the thickness is too thin, it becomesdifficult to maintain strength and distance uniformity between thesubstrates, and when the thickness is too thick, vividness and contrastas a display capability degrade, and in particular, flexibility in thecase of using for an electronic-paper deteriorates.

In the image display device according to the invention, an electrode maybe arranged on the substrate according to need.

In the case of arranging no electrode on the substrate, the eachparticle charged in a predetermined characteristic and having a color ispulled in or rebounds with respect to the substrate by means of anelectric field generated by applying an electrostatic latent image on anouter surface of the substrate. Then, the particle aligned in accordancewith the electrostatic latent image is observed from outside of thedisplay device through the transparent substrate. In this case, theelectrostatic latent image mentioned above can be generated for exampleby a method wherein an electrostatic latent image generated in a knownelectrophotography system using an electrophotography photo-conductor istransferred and formed on the substrate of the image display deviceaccording to the invention, or, by a method wherein an electrostaticlatent image is directly formed on the substrate by an ion flow.

In the case of arranging an electrode on the substrate, the powdercharged in a predetermined characteristic and having a color is pulledin or rebounds with respect to the substrate by means of an electricfield generated on respective electrodes formed on the substrate byapplying an outer voltage thereto. Then, the powder aligned inaccordance with the electrostatic latent image is observed from outsideof the display device through the transparent substrate.

In this case, the electrode may be formed of electroconductive materialswhich are transparent and having pattern formation capability. As suchelectroconductive materials, metals such as aluminum, silver, nickel,copper, and gold, or transparent electroconductive metal oxides such asITO, electroconductive tin oxide, and electroconductive zinc oxideformed in the shape of thin film by sputtering method, vacuum vapordeposition method, CVD (Chemical Vapor Deposition) method, and coatingmethod, or coated materials obtained by applying the mixed solution ofan electroconductive agent with a solvent or a synthetic resin binderare used.

Typical examples of the electroconductive materials include cationicpolyelectrolyte such as benzyltrimethylammonium chloride,tetrabutylammonium perchlorate, and so on, anionic polyelectrolyte suchas polystyrenesulfonate, polyacrylate, and so on, or electroconductivefine powders of zinc oxide, tin oxide, or indium oxide. Additionally,the thickness of the electrode may be suitable unless theelectroconductivity is absent or any hindrance exists in opticaltransparency, and it is preferable to be 3 to 1000 nm, more preferableto be 5 to 400 nm. The foregoing transparent electrode materials can beemployed as the opposed electrode, however, non-transparent electrodematerials such as aluminum, silver, nickel, copper, and gold can be alsoemployed.

In this case, the applied outer voltage may be superimposed with adirect current or an alternate current.

It is preferred that an insulation coating layer is formed on theelectrode so as not to reduce charges of the charged particles. It isparticularly preferred to form this coating layer by a resin havingpositive electrostatic property for the negatively charged particles andby a resin having negative electrostatic property for the positivelycharged particles, since the charges of the particles are difficult tobe reduced.

As to partition walls, it is preferable to form partition walls aroundeach display element. The partition walls may be formed in two paralleldirections. By this structure, unnecessary particles movement in thedirection parallel with the substrate is prevented. Further, durability,repeatability and memory retention are assisted. At the same time, thedistance between the substrates is made uniform as reinforcing thestrength of an image display panel.

The formation method of the partition walls is not particularlyrestricted, however, a screen printing method wherein pastes areoverlapped by coating repeatedly on a predetermined position by screenplate; a sandblast method wherein partition materials are painted with adesired thickness entirely over the substrate and then after coatingresist pattern on the partition materials which is wanted to be left asa partition, jetting abrasive to cut and remove partition materialsaside from the partition part; lift-off method (additive method) whereina resist pattern is formed on the substrate using photopolymer, and thenafter burying paste into a resist recess, removing the resist;photosensitive paste method wherein the photosensitive resin compositioncontaining the partition materials is applied over the substrate andthen obtaining a desired pattern by exposure & developing; and moldformation method wherein paste containing the partition materials isapplied over the substrate and then forming a partition by compressionbonding & pressure forming the dies having rugged structure; and so onare adopted. Further, modifying the mold formation method, reliefembossing method wherein a relief pattern provided by a photopolymercomposition is used as a mold is also adopted.

As to the particles, it is preferred to use spherical particles due toits fluidity.

The average particle diameter d(0.5) is preferable to be 0.1 to 50 μm,particularly to be 1 to 30 μm. When the average particle diameter isless than this range, charge density of the particles will be so largethat an imaging force to an electrode and a substrate becomes toostrong; resulting in poor following ability at the inversion of itselectric field, although the memory characteristic is favorable. On thecontrary, when the average particle diameter exceeds this range, thefollowing ability is favorable, but the memory characteristic willdegrade.

Although the method for charging the particles negatively or positivelyis not particularly limited, a corona discharge method, an electrodeinjection-charge method, a friction charge method and so on areemployable.

It is preferable that the absolute value of the difference between thesurface charge densities of the particles, which are measured by ablow-off method using a carrier, is 5-150 μC/g as the absolute value.When the absolute value of the surface charge density is less than thisrange, response speed to the change of an electric field will be late,and the memory property degrades. When the absolute value of the surfacecharge density exceeds this range, image force for the electrode or thesubstrate will be so strong that the memory property will be favorable,but following ability will be poor in the case where the electric fieldis inverted.

Because it is necessary for the particles to hold the charged electriccharge, insulating particles with the volume specific resistance of1×10¹⁰ Ω·cm or greater are preferable, and in particular, insulatingparticles with the volume specific resistance of 1×10¹² Ω·cm or greaterare more preferable.

In the image display device in which the particles are flown and movedby Coulomb's force etc. so as to display the image as mentioned above,the feature of the invention is to use the particles for the imagedisplay having micro-irregularity on their surfaces. Since themicro-irregularity is arranged on the surface of the each particle, thefollowing properties can be obtained.

(As to Drive Voltage)

As a large factor for applying influence to the drive voltage, there arean intermolecular force between the particle and the electrode plate anda liquid bridging force. If the micro-irregularity is arranged on asurface of the particle, these adhesion forces are largely decreased,and the drive voltage can be lowered.

(As to Charge Amount of Particles)

If the micro-irregularity is arranged on the surface of the eachparticle, a specific surface area per single particle can be increased.Therefore, chargeable sites are increased, and thus it is possible toobtain the particle having a sufficient electrostatic property.Moreover, the micro-irregularity causes a charge concentration at itsportion, and thus it is possible to obtain the each particle having amore sufficient electrostatic property.

(As to Whiteness)

If the micro-irregularity is arranged on the surface of the eachparticle, an incident light causes a irregular reflection, and thus theparticle shows a white color when viewing.

In the present invention, a specific surface area per each particle isincreased by arranging the micro-irregularity on the surface of the eachparticle, but it is preferred that a product of a specific surface areaS (m²/g) and an average particle diameter d(0.5) (μm) of the particle:(S×d(0.5)) is 10 or more.

The average particle diameter d(0.5) is obtained from the particlediameter distribution and means a value of the particle diameterexpressed by μm wherein an amount of the particles having the particlediameter larger than or smaller than this value is 50%.

Here, the particle diameter distribution and the average particlediameter mentioned above can be measured by means of a laserdiffraction/scattering method. When a laser light is incident upon theparticles to be measured, a light intensity distribution pattern due toa diffraction/scattering light occurs spatially. This light intensitydistribution pattern corresponds to the particle size, and thus it ispossible to measure the particle diameter and the particle diameterdistribution.

In the present invention, it is defined that the average particlediameter and the particle diameter distribution are obtained by a volumestandard distribution. Specifically, the average particle diameter andthe particle diameter distribution can be measured by means of ameasuring apparatus Mastersizer 2000 (Malvern Instruments Ltd.) whereinthe particles setting in a nitrogen gas flow are calculated by aninstalled analysis software (which is based on a volume standarddistribution due to Mie's theory).

The method for arranging the micro-irregularity on the surface of theeach particle is not particularly limited, and, for example, thefollowing methods can be used.

-   1) Porous particles are produced by including a volatile component    in an inner portion of the particles at polymerization and removing    the volatile component by heating after polymerization. In this    manner, it is possible to arrange the micro-irregularity on each    surface of the particles.

Specifically, a solvent having a boiling point higher than a reactiontemperature at suspension polymerization such as toluene and xylene isincluded in an oil spot at polymerization, and the volatile component isremoved by heating after polymerization, so as to obtain the porousparticles.

-   2) The micro-irregularity is formed on each surface of the particles    by crashing or sliding a substance having a high stiffness with    respect to each surface of polymerized particles.

Specifically, use is made of a sandblast method or a method wherein thepolymerizes particle is filled in a vessel together with a metal powderand so on, which is further shaken by means of a paint-shaker, and afterthat only the polymerized particles are picked up by means of a sieve,so as to obtain the particles having the micro-irregularity on theirsurfaces.

-   3) The polymerized portion having an indefinite shape is arranged on    each surface of polymerized particles obtained by a suspension    polymerization by means of a graft polymerization method.

For example, as described in Japanese Patent Laid-Open Publication No.8-114947, it is possible to obtain the particles having themicro-irregularity by polymerizing a monomer on each surface of mainparticles under a water system including the main particles, apolymerizable monomer, a dispersant having a skeleton of thepolymerizable monomer and a soluble initiator.

-   4) The surface area is enlarged by adhering small child particles on    mother particles.

This treatment is performed by adding the child particles in a solutionobtained by dissolving the charge control agent into the solvent,separating the child particles by means of a filtration and drying thefiltered child particles.

As the child particles, use is made of minute particles of metal oxidesuch as silica, titanium oxide. If a surface treatment is performed withrespect to the minute particles by using the charge control agent, thecharge control agent is fixed to each surface of the minute particles toshow a white color, and it is possible to charge the particles in adesired property.

The charge control agent used for adhering the child particles on eachsurface of the mother particles is not limited if it is soluble in asolvent and its charge can be controlled, and thus any charge controlagent available in the market may be preferably used.

For example, use is made of nigrosine compound, resin acid modifiedazine, resin acid modified azine compound, the fourth grade ammoniumsalt, salicylic acid metal complex, phenol condensate, metal-containingazo dye, and triphenylmethane derivative.

Moreover, it is possible to control dyeing to a black color or aneggplant color at the same time of the charge control operation byselecting the charge control agent to be used, and thus the minuteparticles for display having a black color can be obtained.

That is, among the above charge control agents, it is possible to dyethe particle by using a solution in which nigrosine compound, resin acidmodified azine, resin acid modified azine compound, or metal-containingazo dye is dissolved.

Specifically, use is made of negative charge control agent such assalicylic acid metal complex, metal-containing azo dye, oil-soluble dyeof metal-containing (containing a metal ion or a metal atom), the fourthgrade ammonium salt-based compound, calixarene compound,boron-containing compound (benzyl acid boron complex), andnitroimidazole derivative. Moreover, use is made of positive chargecontrol agent include nigrosine dye, triphenylmethane compound, thefourth grade ammonium salt compound, polyamine resin, imidazolederivatives, etc.

Additionally, as the charge control agent, use is made ofnitrogen-containing circular compound such as pyridine, and so on, andthese derivates or salts; and resins containing various organicpigments, fluorine, chlorine, nitrogen, etc.

As the solvent, use is made of any solvent if it dissolves the chargecontrol agent and shows no expansion and no dissolution of the minuteparticles, and normally it is preferred to use alcohol.

The treatment method is performed by adding 0.1-10% of the chargecontrol agnet in the solvent, and agitating and dissolving them by meansof a mixer and so on. The thus obtained solvent is subjected to afiltration so as to remove an undissolved component, and the minuteparticles is added in the filtered solvent which is further agitated bymeans of the mixer and so on. The minute particles treated by thefiltration is picked up from the mixed solution and the picked-up minuteparticles is dried, so that the particle for the image display can beobtained.

The charge control agent and coloring agent can be used at the sametime. As for a coloring agent, various kinds of organic or inorganicpigments or dye as will be described below are employable.

Examples of black pigments include carbon black, copper oxide, manganesedioxide, aniline black, and activate carbon.

Examples of yellow pigments include chrome yellow, zinc chromate,cadmium yellow, yellow iron oxide, mineral first yellow, nickel titaniumyellow, navel orange yellow, naphthol yellow S, hanzayellow G,hanzayellow 10G, benzidine yellow G, benzidine yellow GR, quinolineyellow lake, permanent yellow NCG, and tartrazinelake.

Examples of orange pigments include red chrome yellow, molybdenumorange, permanent orange GTR, pyrazolone orange, Balkan orange,Indanthrene brilliant orange RK, benzidine orange G, and Indanthrenebrilliant orange GK.

Examples of red pigments include red oxide, cadmium red, diachylon,mercury sulfide, cadmium, permanent red 4R, lithol red, pyrazolone red,watching red, calcium salt, lake red D, brilliant carmine 6B, eosinlake, rhodamine lake B, alizarin lake, and brilliant carmine 3B.

Examples of purple pigments include manganese purple, first violet B,and methyl violet lake.

Examples of blue pigments include Berlin blue, cobalt blue, alkali bluelake, Victoria blue lake, phthalocyanine blue, metal-free phthalocyanineblue, partially chlorinated phthalocyanine blue, first sky blue, andIndanthrene blue BC.

Examples of green pigments include chrome green, chromium oxide, pigmentgreen B, Malachite green lake, and final yellow green G.

Examples of extenders include baryta powder, barium carbonate, clay,silica, white carbon, talc, and alumina white. Furthermore, there areNigrosine, Methylene Blue, rose bengal, quinoline yellow, andultramarine blue as various dyes such as basic dye, acidic dye,dispersion dye, direct dye, etc. These coloring agents may be used aloneor in combination of two or more kinds thereof.

The thus surface-treated minute particles (child particles) are fixed tothe each mother particle as a core, and the electrostatic propertyapplying and the coloring can be performed.

As the fixing method, use may be made of a wet-type treatment method inwhich the child particles are coated on each surface of the motherparticles together with binder resin. However, in this wet-typetreatment method, it is necessary to select the solvent so as to preventa re-dissolving of the charge control agent. Therefore, it is preferredto fix the child particles to the each mother particle by using adry-type treatment method from the viewpoint of simplicity.

As a device for the dry-type treatment, Hybridizer (Nara Machinery Co.,Ltd.) and MechanoFusion (Hosokawa Micron Co., Ltd.) are well known. Theyare different type, but they are preferably used.

It is preferred to set the ratio (d₁/d₂) between the average particlediameter d₁ of the mother particles and the average particle diameter d₂of the child particles to 10 or more. The ratio d₁/d₂ is normally 100 orless. If the ratio d₁/d₂ is less than 10 (that is, d₂ is larger), thenumber of the child particles to be fixed becomes smaller, and thus itis not possible to obtain the effects of the present invention.

The mother particles may be preferably a circular shape and can beproduced by performing the polymerization from monomer. Moreover,according to need, a classifying operation is performed for controllingthe particle size. Further, in addition to this, the particle can beobtained by crashing and classifying the resin.

Typical examples of the resin include urethane resin, urea resin,acrylic resin, polyester resin, acryl urethane resin, acryl urethanesilicone resin, acryl urethane fluorocarbon polymers, acryl fluorocarbonpolymers, silicone resin, acryl silicone resin, epoxy resin, polystyreneresin, styrene acrylic resin, polyolefin resin, butyral resin,vinylidene chloride resin, melamine resin, phenolic resin, fluorocarbonpolymers, polycarbonate resin, polysulfon resin, polyether resin, andpolyamide resin. For the purpose of controlling the attaching force withthe substrate, acryl urethane resin, acryl silicone resin, acrylfluorocarbon polymers, acryl urethane silicone resin, acryl urethanefluorocarbon polymers, fluorocarbon polymers, silicone resin areparticularly preferable. Two kinds or more of these may be mixed andused.

The distance between the transparent substrate and the opposed substrateis suitably adjusted in a manner where the particles can move andmaintain the contrast of image display; however, it is adjusted usuallywithin 10 to 5000 μm, preferably within 30 to 500 μm.

The volume population of the particles existing in the space between thefaced substrates is preferable to be 10 to 80%, more preferable to be 10to 70%.

The image display device according to the invention is applicable to theimage display unit for mobile equipments such as notebook personalcomputers, PDAs, cellular phones and so on; to the electric-paper forelectric book, electric-newspaper and so on; to the bulletin boards suchas signboards, posters, blackboards and so on; and to the image displayunit for electric calculator, home electric application products, autosupplies and so on.

Then, the present invention will be explained in detail with referenceto the examples. However, the present invention is not limited to thefollowing examples.

In the following example and comparative example, measurements of theaverage particle diameter d(0.5) and the surface charge density wereperformed as follows.

(1) Average Particle Diameter d(0.5) (μm)

Respective particle was set in an apparatus for measuring a particlediameter distribution (Mastersizer2000, Malvern instruments Ltd.), andthe particle diameter distribution was measured. Then, by using theattached analysis software, the average particle diameter d(0.5) (μm),which was a value of the particle diameter expressed by μm wherein anamount of the particles having the particle diameter larger than orsmaller than this value was 50%, was determined.

(2) Specific Surface Area S (m²/g)

It was measured according to BET method.

(3) Surface Charge Density

<Blow-Off Measuring Theory and Method>

In the blow-off method, a mixture of the particles and the carriers areplaced into a cylindrical container with nets at both ends, andhigh-pressure gas is blown from the one end to separate the particlesand the carriers, and then only the particles are blew off from the meshof the net. In this occasion, charge amount of reverse polarity remainson the carriers with the same charge amount of the particles carriedaway out of the container. Then, all of electric flux by this electriccharge are collected to Faraday cage, and are charged across a capacitorwith this amount. Accordingly, the charge amount of the particles isdetermined as Q=CV (C: capacity, V: voltage across both ends of thecapacitor) by measuring potential of both ends of the capacitor.

As a blow-off powder charge amount measuring instrument, TB-200 producedby Toshiba Chemical Co., Ltd. was used. Two kinds of positivelychargeable and negatively chargeable carrier-particles were employed asthe carriers, and charge density per unit area (unit: μC/m²) wasmeasured in each case. Namely, F963-2535 available from Powder TEC Co.,Ltd. was employed as a positive chargeable carrier-particle (the carrierwhose opponent is positively charged and itself tends to be negative)and F921-2535 available from Powder TEC Co., Ltd. was employed asnegatively chargeable carrier-particle (the carrier whose opponent isnegatively charged and itself tends to be positive). The surface chargedensity of the particles was obtained from the measured charge amount,the average particle diameter and specific gravity of the particlesmeasured separately.

<Particle Specific Gravity Measuring Method>

The specific gravity was measured with the use of a hydrometer producedby Shimadzu Seisakusho Ltd. (brand name: Multi volume Density MeterH1305).

(4) Estimation of Display Function

The black/white display was repeated by repeatedly inversing a potentialof 500 V applied to the assembled display device. The estimation of thedisplay function was performed by measuring displays of white color andblack color by using a reflection image densitometer (RD918, MacbethCo., Ltd.). In this case, a contrast ration means a ratio of areflection density when the black color display was performed withrespect to the reflection density when the white color display wasperformed (=reflection density at black color display/reflection densityat white color display).

In this case, unevenness at overall surface display was determined onthe basis of the following standard.

-   ◯: Overall surface showed black color/white color at substantially    100%.-   Δ: In the black display, slight white color portion was included    partly, or, in the white display, slight black color portion was    included partly.-   x: Black color/white color display was mixed considerably.

Moreover, a white color visibility was determined on the basis of thefollowing standard.

-   ◯: Contrast of black color/white color was sufficient, and fine    patterns could be recognized sufficiently.-   Δ: Contrast of black color/white color was slightly insufficient,    but fine patterns could be recognized in any case.-   x: Pattern recognition was difficult since contrast of black    color/white color was insufficient.

EXAMPLE 1

As white color particles, use was made of particles in which 0.2 wt % ofhydrophobic silica (H3004, Hoechst Japan Ltd.) was added in a porouspolymethyl methacrylate particles (MBP8, SEKISUI PLASTICS CO., LTD, theaverage particle diameter d(0.5) of 6.1 μm).

As black color particles, use was made of a circular polymethylmethacrylate particles (Techpolymer MBX-5B, SEKISUI PLASTICS CO., LTD.,the average particle diameter d(0.5) of 5.6 μm).

The display device was produced as follows. That is, a pair of glasssubstrates, on which indium oxide electrode having a thickness of about500 Å (thickness: 50 nm), was assembled in such a manner that aninterval between the substrates was controlled to be 100 μm by usingspacers. Then, the white color particles and the black color particlesmentioned above were filled in the space between the glass substrates,and peripheral portions of the glass substrates were connected by epoxyadhesive, so that the display device was produced. It should be notedthat the mixing rate of the white color particles and the black colorparticles was controlled to be even, and the filling rate of thesesparticles between the glass substrates was controlled to be 50 vol %.The estimation results of the particles properties and the displayfunctions are shown in Table 1.

COMPARATIVE EXAMPLE 1

The image display device was produced in the same manner as that of theexample 1, except that a circular polymethyl the methacrylate particle(MBX8, SEKISUI PLASTICS CO., LTD., the average particle diameter d(0.5)of 5.6 μm) was used as the white color particles instead of the porouspolymethyl methacrylate particles (MBP8, SEKISUI PLASTICS CO., LTD., theaverage particle diameter d(0.5) of 6.1 μm). The estimation results ofthe particles properties and the display functions are shown in Table 1.

TABLE 1 Comparative Example 1 Example 1 (White particles) Material MBP8MBX8 Specific surface area (m²-g) 85 0.8 Average particle diameterd(0.5)(μm) 6.1 5.6 S × d(0.5) 518.5 4.48 Color White Clear Surfacecharge density (μC/m²) 48 33 (Black particles) Material MBX-5B MBX-5BAverage particle diameter d(0.5)(μm) 5.6 5.6 Estimation of displayparticles Image displaying Overall white displaying (A) 0.45 0.75Overall black displaying (B) 1.4 1.4 Contrast ratio (B/A) 3.1 1.9Unevenness at overall displaying ◯ ◯ White color visibility ◯ X

INDUSTRIAL APPLICABILITY

The present invention relates to particles for image displaying used inan image display device, in which one or more kind of particles aresealed between opposed substrates, at least one substrate beingtransparent, and, in which the particles are made to fly and move so asto display an image, characterized in that each particle has amicro-irregularity at its surface. In this invention, since the particlefor the image display mentioned above is used, it is possible to obtainthe image, which can achieve a low electrostatic property, sufficientlyapply an electrostatic property to the particles and stably obtain anexcellent image having a sufficient contrast.

1. Particles for image displaying used in an image display device,comprising particles sealed between opposed substrates, at least one ofsaid opposed substrate being transparent, and, in which the particlesare made to fly and move so as to display an image, characterized inthat each particle has a micro-irregularity at its surface, wherein aproduct of a specific surface area S (m²/g) and an average particlediameter d(0.5) (μm) of the particles: (S×d(0.5)) is 10 or more.
 2. Theparticles for image displaying according to claim 1, wherein the averageparticle diameter d(0.5) is in the range of 0.1-50 μm.
 3. The particlesfor image displaying according to claim 1, wherein the particles areproduced by including a volatile component in an inner portion of theparticle at polymerization and removing the volatile component byheating after polymerization.
 4. The particles for image displayingaccording to claim 1, wherein the particles are produced by crashing orsliding a substance having a high stiffness with respect to each surfaceof polymerized particles so as to form the micro-irregularity on thesurface of the each particle.
 5. The particles for image displayingaccording to claim 1, wherein the particles are produced by arranging apolymerized portion having an indefinite shape on each surface ofpolymerized particles obtained by a suspension polymerization, by meansof a graft polymerization method.
 6. The particles for image displayingaccording to claim 1, wherein the particles are produced by adheringsmall child particles on each surface of large mother particles.
 7. Animage display device, in which one or more kind of particles are sealedbetween opposed substrates, at least one substrate being transparent,and, in which the particles, to which an electrostatic field produced bytwo kinds of electrodes having different potentials is applied, are madeto fly and move so as to display an image, characterized in that use ismade of each particle having a micro-irregularity at its surface,wherein a product of a specific surface area S (m²/g) and an averageparticle diameter d(0.5) (μm) of the particles: (S×d(0.5)) is 10 ormore.
 8. An image display device comprising: two opposed substrates;particles, which are sealed between said opposed substrates; whereinsaid particles are configured to move so as to display an image; whereinat least one of said two opposed substrates is substantiallytransparent; wherein a surface of each of said particles has amicro-irregularity; and wherein a product of a specific surface area S(m²/g) and an average particle diameter d(0.5) (μm) of said particles:(S×d(0.5)) is 10 or more.